1. Field of the Invention
The present invention relates to a solid-state imaging apparatus and a method for driving a solid-state imaging device, in which light received by a plurality of two-dimensionally arranged photoelectric conversion portions is converted into electric signals and outputted as image signals. In particular, the present invention relates to a solid-state imaging apparatus and a method for driving a solid-state imaging device, in which the image signals can be outputted at a high frame rate.
2. Description of Related Art
Known solid-state imaging devices, which convert light received by individual pixels into electric signals and output them as image signals, include those using CCDs (charge-coupled devices). Also, digital still cameras utilizing such solid-state imaging devices have been widespread. In recent years, with an increase in the density of pixels in the solid-state imaging devices, the digital still cameras achieving a higher resolution than silver halide photographs have been achieved. FIG. 7 shows the configuration of a general single-plate color solid-state imaging device for a conventional digital still camera. This solid-state imaging device 101 includes photoelectric conversion portions 102 having color filters in a Bayer arrangement, readout portions 103 for reading out signal charges respectively from the photoelectric conversion portions 102, vertical transfer portions 104 that are arranged so as to correspond to respective columns of the photoelectric conversion portions 102 for transferring the read out signal charges in a vertical direction, a horizontal transfer portion 105 for transferring the signal charges received from the vertical transfer portions 104 in a horizontal direction, and an output portion 106 for amplifying and outputting the signal charges from the horizontal transfer portion 105.
In many cases, the digital still cameras have a function of recording moving pictures as well as still pictures. Most of the digital still cameras now have more than 4,000,000 pixels, for example, for recording still pictures, whereas the pixels generally are thinned out so as to secure the frame frequency necessary for recording moving pictures (for example, at least 30 frames/second). In a generally adopted method for thinning out (reducing the number of) the pixels in the vertical direction, the signal charges in not all but only part of the photoelectric conversion portions in each column, for example the signal charge of one of the three adjacent photoelectric conversion portions, is selected and read out to the vertical transfer portion.
Another method for reducing the number of the pixels in the vertical direction is described in JP 9(1997)-18792 A. In this method, the signal charges at adjacent plural vertical transfer stages among plural vertical transfer stages constituting the vertical transfer portion are transferred successively to the horizontal transfer portion. In this way, the signal charges at the adjacent plural vertical transfer stages are mixed together in the horizontal transfer portion so as to reduce the number of the pixels in the vertical direction, thereby making it possible to achieve a still higher frame frequency.
Further, JP 2004-180284 A describes a solid-state imaging device capable of reducing the number of (thinning out) pixels in the horizontal direction. In this solid-state imaging device, vertical last stages of the individual columns of the vertical transfer portions have transfer electrodes with identical configurations repeated every (2n+1) columns (for example, three columns) in which at least two independent transfer electrodes that are independent of the transfer electrodes in the other columns in the (2n+1) columns for controlling a transfer operation from the vertical last stage to the horizontal transfer portion by each column. For example, in the case where two colors of pixels alternate in a single row as in the Bayer arrangement, the operation of transferring selectively the signal charges in every other pixels with the same color from the vertical last stages to the horizontal transfer portion and mixing them together is repeated (2n+1) times, thereby reducing the number of the pixels in the horizontal direction to 1/(2n+1).
As described above, when the moving pictures are recorded using a solid-state imaging device with a large total number of pixels, the number of pixels is reduced in such a manner as not to lower the frame frequency. In this case, for suppressing image quality degradation, it is desired to reduce the number of pixels in both of the horizontal and vertical directions so that the resolutions in the horizontal and vertical directions are balanced. However, it would not be possible to combine the configuration described in JP 9(1997)-18792 A capable of reducing the number of pixels in the vertical direction with that described in JP 2004-180284 A capable of reducing the number of pixels in the horizontal direction. In other words, it would be impossible to perform the operation in which the signal charges at plural vertical transfer stages are transferred successively to the horizontal transfer portion as in the configuration described in JP 9(1997)-18792 A simultaneously with the operation in which the signal charges in every other pixels with the same color in the horizontal direction present in the vertical last stages are mixed together in the horizontal transfer portion as in the configuration described in JP 2004-180284 A. Accordingly, in the case of reducing the number of pixels in the vertical direction simultaneously with reducing the number of pixels in the horizontal direction with the configuration described in JP 2004-180284 A, dummy vertical transfer stages from which the signal charges in the photoelectric conversion portions are not read out at all (dummy transfer stages) are formed partially so that the dummy transfer to the horizontal transfer portion is performed, thereby reducing the number of pixels in the vertical direction.
However, in this case, there is a problem that a smear occurring when capturing an image of a radiant with too much light quantity has a jagged edge, though a smear naturally has a linear edge. For example, in the case where signal charges from three pixels with the same color, which are every other pixels arranged in the horizontal direction, are mixed together in the configuration of JP 2004-180284 A, pixel signals of the three vertical transfer stages are aligned periodically in the horizontal transfer portion in the order of G1, G2, G3, R1, B2 and R3 (R, G and B respectively indicate red, green and blue, and numerals 1, 2 and 3 indicate the first, second and third vertical transfer stages from the side closer to the horizontal transfer portion). These pixel signals should be rearranged at their original mixture centers of gravity of the respective pixels at the time of displaying an image. However, in the configuration of JP 2004-180284 A, while the charge signals in the horizontal transfer portion are being shifted in the horizontal direction, the charge signals at the vertical last stages are transferred to the horizontal transfer portion and mixed together. Thus, smear charges at the dummy transfer stages are mixed with the signal charges at the vertical transfer stages in the other columns. As a result, in the image rearranged at the original mixture centers of gravity, the smear edge, which is naturally linear in the vertical direction, is displaced by each set of three pixels in the vertical direction and thus appears jagged.
Further, even in the case where no smear occurs, if a transfer leakage from the vertical transfer stage that is transferring a signal charge causes the signal charge to leak into the dummy transfer stage during transfer through the vertical transfer stages, the leaking signal charge sometimes is mixed with the signal charge in another column in the horizontal transfer portion, which may result in poor image quality due to transfer degradation.